Generally, a belt-driven power-assist parallel hybrid vehicle designates a vehicle configured wherein the mechanical power transmitted to a drivetrain is generated by both an engine and an electric motor in order to maximize fuel economy. The electric motors used in these hybrid vehicles are typically of the squirrel-cage induction and permanent magnet types.
One of the attractive features of using a permanent motor in a belt-driven parallel hybrid vehicle as a secondary source of propulsion is that it offers high efficiency during periods of utilization in low speed urban or city driving that offsets its magnetic drag losses due to the rotating permanent magnets during periods of non-utilization. However, during moderate to high speeds in a drive cycle wherein the permanent magnet motor is not being utilized, the rotating magnetic drag losses increase the resistance of the drivetrain that grossly exceeds the benefits of high efficiency, and as a consequence reduced fuel efficiency is achieved.
Therefore, one disadvantage of utilizing a permanent magnet motor in a belt-driven power-assist parallel hybrid vehicle is that it adds unnecessary magnetic drag losses to the drivetrain during periods of the drive cycle when is not being utilized. Thus, it is not desirable in a belt-driven power-assist parallel hybrid vehicle where electric motor utilization is low. In addition, the additional magnetic drag losses due to the rotating permanent magnets are converted into heat that require increased heat rejection capability of the cooling system in the vehicle.
On the other hand, using an induction motor as a secondary source of propulsion in a belt-driven power-assist parallel hybrid vehicle eliminates the rotating magnetic drag losses that inherently exist in permanent magnet motors, and this can result in improvement in fuel efficiency. Induction motors are desirable for many traction applications due to their unique smooth torque-speed characteristics as well as capability to handle high-to-low and low-to-high torque transitions smoothly. Induction motors have a unique performance advantage over permanent magnet motors in terms of transient overload capability. That is, for a given current higher transient overload torque and power capability can be achieved with an induction motor since its torque is a function of the square of the available current than a permanent magnet motor whose torque is a function of current. Short transient overload capability is an important requirement that an electric motor must meet in a hybrid vehicle application as a secondary source of propulsion. This capability combined with careful design, cost, ruggedness and reliability are some of the reasons induction motors are utilized extensively in traction applications to provide high torque at relatively high efficiency. In addition to their primary use as motors, they may be used as generators which may be operated to apply regenerative brake to a hybrid vehicle as needed for conversion of mechanical energy extracted during braking or deceleration to electrical power for charging the onboard electric energy storage device.
However, utilizing an induction motor in regenerative mode as a generator to provide electrical power is not one of its strong features. The disadvantage is due to the fact that in generate mode an induction motor operates with a lagging power factor, and therefore consumes reactive power that must be supplied to magnetized its magnetic circuit for proper operation. As a result, induction motors have lower efficiency when operated in generator mode than motor mode.
Improving the fuel efficiency of a hybrid vehicle throughout its drive cycle requires: (1) that the electric energy supplied by the onboard electric energy storage device during motoring mode must be efficiently converted to mechanical power to provide torque- and power-assist to the drivetrain and wheels, and (2) that the mechanical energy extracted from the drivetrain and wheels during regenerative braking mode must be efficiently converted to electrical power to charge the onboard electric energy storage device. This means that the power losses, which are dissipated in the electric motor during the power conversion process from electrical power to mechanical power and vice versa, and resulting heating and temperature rise must be minimize throughout the drive cycle from startup to high speed in order to achieve improved fuel efficiency.
A very small number of disclosures on combined motor-generator constructions are known in the prior arts for hybrid vehicle and aerospace applications, such as U.S. Pat. No. 4,476,395, U.S. Pat. No. 7,389,837, WO2011106944, and U.S. Pat. No. 8,360,181, and the majority of them are invariably limited to aerospace applications. In particular, a combination induction generator/permanent magnet generator in a single housing was disclosed in U.S. Pat. No. 4,476,395 as a source of electrical power generation in aircrafts. However, the invention was directed to the use of a tandem induction generator/permanent magnet generator combination for the role particularly limited to electrical power generation in aircrafts. The invention described herein differs from prior arts in the construction and method of control that overcome the drawbacks discussed above in order to achieve the objective of the present invention, which is to improve fuel efficiency of a belt-driven parallel hybrid vehicle.
Therefore, there is a need to provide a starter-generator for a hybrid vehicle application that converts: (1) the electric energy supplied by the onboard electric energy storage device more efficiently to provide mechanical power that is transmitted to the drivetrain and wheels, and (2) the mechanical energy captured during regenerative braking more efficiently to electrical power for charging the onboard electric energy storage device.
Accordingly, it is an object of this present invention to provide a starter-generator construction that combines an induction motor-generator that operates mainly in motoring mode and a permanent magnet motor-generator that operates mainly in generating mode in a compact single housing enclosure so that it easily mounts as a belt-starter alternator (BAS) for the benefits of further improving fuel efficiency of the prior art belt-driven parallel hybrid vehicle, and which overcomes the disadvantages of the induction motor-generator in generative mode and permanent magnet motor-generator magnetic drag losses during periods of non-utilization at moderate to high speeds in a drive cycle of a hybrid vehicle.